Transmission
From Organic Design wiki
Transmission is the default P2P torrent downloading application that comes with Ubuntu.
Starting and stopping on schedule
To run transmission from the crontab (/etc/crontab), it must be associated with a display as in the following example. The first line starts transmission at midnight, and the second stops it at 8am. Edit the crontab with sudo nano /etc/crontab.
0 0 * * * username transmission --display=:0 0 8 * * * username transmission --display=:0 --quit
